Voltage compensation



Jllrl l, W- H. HOWE VOLTAGE COMPENSATION Filed NOV. 29, 1933 4SheetS-Sheet l Jun@ 1, w37. WHHOWE 2,082,497

VOLTAGE COMPENSTION Filed Nov. 29A bw 4 Sheets-Sheet 2 j@ Z ff. W

June l, 1937. W H HQWE 2,082,497

VOLTAGE GOMPENSAT ION Filed Nov. 29, 1933 4 Sheets-Sheet I5 gy/ W. E Z.XW @if June 1, 19312 w.' H. HOWE 2,082,497

VOLTAGE GOMPENSATIN FiledNOv. 29, 1953 4 SheetS-Sheet 4 if f JZ Jf (E)/M ,mm

Patented June 1, 1937 l uNrrl-:D STATES PATENT OFFlCE VOLTAGECOMPENSATION Application November 2t", 1933, Serial No. 700,234

14 Claims.

.This invention relates to a method and circuits for compensating forvariations in a source of power, morev particularly when that source isused through electrical means to energize mechanism, the response ofwhich it is desired to make correspond to variations in a variableelectrical characteristic and whereby such source variations shall besubstantially or wholly without ef lect on the mechanism. Such a sourcemay, for

0 example, be a commercial source of electrical power, which, as isweHknown, is subject to voltage variations and iluctuations, often sudden,and which if not compensated for would disturb the response or themechanism energized therel from which it is desired shall be responsiveonly to variations in the variable controlling characteristic. Thesource may, however, be a diierent. source subject to power variationswhich give rise to corresponding variations in voltage in associatedcircuits. The system of the invention acts to compensate lor suchvoltage variations from whatever cause they may arise.

In general the compensative system of this invention comprises threeportions: (a) a circuit which, for convenience, may be called ameasuring circuit, including a variable impedance or other electricalcharacteristic to be measured, or the variations oi which are employedfor control or other purposes, andmeans responsive in a mannercorresponding to variations in the variable characteristics; (b) a powerconversion or control unit interposed between the power supply and themeasuring circuit and having an elec-l trical characteristic variablewith variations of the power supply; and (c) a compensating connectionwhereby the changes in the electrical characteristic in the powerconversion or control unit are fed to the responsive means in such amanner that variations in the responsive means arising from variationsin the power supply are compensated for so that such power supplyvariations do not aiect the response in the meter circuit, at least whenthe desired response indication is being given. This power conversion orcontrol unit often, though not necessarily, consists oi apparatus whoseprincipal function is to receive power as conveniently available (say,105 to 120 volts 60 cycles from the lighting circuit) and to convertthis power to the form necessary for the operation of the measuringcircuit (say 366 to 330 volts 1500 kilocycles for the operation of acapacity measuring circuit).

For a more complete understanding of this invention, reference may behad to the accom panying drawings in which Figure 1 is a diagram oi aresistance measuring circuit to which voltage compensation in accordance with this invention may be applied.

Figure 2 is a current voltage curve for the bal last tube used in thecircuit of Figure 1. I Figure 3 is a diagram showing the eilects of linevoltage variation in a circuit such as shown in Figure 1.

Figure 4 is a diagram of the circuit of Figure 1 to which has been addedline voltage variation l0 compensation in accordance with thisinvention.

Figures 5 and 6 are diagrams showing the performance of the circuit ofFigure 4 with various values of certain electrical characteristics.

Figure 7 is a diagram like Figure 4 except that 15 the parts arediierently placed.

Figures 8, 9, and l0 are diagrams of other types of measuring circuitsincluding the voltage variation compensation of this invention.

Figure l1 is a diagram somewhat similar to 2o Figure s, but showing acontrolmechanism rather than an indicator.

Figure 12 is a view somewhat similar to Figure 9, but showing apotentiometer type oi' indicating circuit. 25

Figure 1 illustrates a measuring circuit for the measurement of aresistance Rm. 'Ihe current flow through this resistance is measured bythe meter M, which thereby varies with the resistance provided aconstant voltage is applied. In order 30 to obtain a constant voltage,va voltage regulator shown as a ballast tube I0 is interposed betweenthe supply lines Il and the measuring circuit. The regulating effect orsuch a ballast tube depends upon the variation of resistance of an ironwire in an atmosphere of hydrogen with variations of temperature of theiron wire due to change of current iiow.

The current voltage curve for a ballast tube is shown in Figure 2. Thiscurve shows that within the working range from 35 to 65 volts across thetube a relatively small change in voltage is accompanied by a largechange in current. 'Ihe performance within the working rangeapproximates the following equation: 45

Voltage it@ current Q05 in the circuit of Figure 1 there is a parallelresistance Re connected in series with the tube and in parallel with theload, the value oi' this re- 50 sistance seing such that the currentwithin the tube lies within this working range.

Figure 3 shows the performance ci the circuit of Figure l. with such avoltage regulator with a total resistance of Pe in parallel with Rm cit@ 5g ohms. The upper full line curve i5 shows the output voltage acrossthe load and the curve i6 the voltage across the ballast tube both beingplotted against line voltage. The dash line I1 indicates 5 the voltagevariation `which would exist if the resistance of the ballast tube wereconstant. A nparison of this curve with the curve I5 indieates theeffects of the voltage regulator to reduce voltage variation on themeter resulting l0 from variations of line voltage. It will be noted,

however,'that the curve I5 indicates a considerable uncorrected errorwhich would give rise to considerable possible error in the meterreading due to changes of line voltage. Figure 4 illustrates a circuitof the type shown in Figure l, but provided with a feed-back circuit inaccordance with this invention. This feed-back or compensating circuitconnects a nxed resistance Rs direct from the supply line to themeasuring resistance Rm in parallel with the ballast tube and meter M.This resistance Rs is large in comparison with that of the ballast tube,so that the current through it is small in comparison with that throughthe ballast tube. However, since the resistance R5 is constant, whilethat of the ballast tube increases rapidly with increase of supplyvoltage, the proportion of current through Rr increases more than inproportion to the increase of line voltage. By a proper choice of theresistance of R, this more than proportional increase of current fiowthrough Rs occurring with an increase of supply voltage can be madeexactly equal in amount to the increase of current through Rm resultingfrom this same increase of supply voltage incompletely regulated by theballast tube. Under these circumstances, there will be no change in thecurrent flow through meter M with change of line voltage, i. e. meterreading is rendered independent of supply voltage changes.

Figure 5 shows the performance of the circuit of Figure 4 with theresistance R=50 ohms, Rm=200 ohms, and various values of the feedbackcircuit resistance R., the meter current Im being plotted against theline voltage E1. It will be noted that in the upper curve of thisfigure, R. equals infinity, which represents the conditions of Figure 1and corresponds to the curve l5 in Figure 3. It will be noted that witha resistance R.. of the value of 600 ohms exact compensation is producedso that the meter reading is independent of line voltage variations.

Figure 6 is the same as Figure 5, except that the measuring resistanceRm has been changed from 200 to 150 ohms, thus shifting the level of themeter current and slightly altering the curve shapes. In this conditionexact compensation for line voltage variations exists with R5=450 ohms.In general it may be shown that perfect compensation will be obtainedwhen -where K is the constant of the ballast tube. The equation of thevoltage and the current of the ballast tube within its working range isEzK-Q from which the constant K may be obtained, the performance withinthe working range as heretofore mentioned being approximately with valueof 150 for K, and the constant Q being approximately 205. It should benoted that exact cornpensation exists only with a certain value of R5for any value of Rm but through a considerable range of values of Rm, anaverage value or Rg provides very great improvement of voltage com--pensation so that a very nearly perfect compensation land a veryconsiderable improvement over the condition where R. equals infinity,that is, where there is no compensating feed-back circuit, is obtained.

The circuit of Figure 4 in some ways resembles a plain bridge circuit,the parts being arranged as in a bridge circuit in Figure 7. Figure 'Iis exactly like Figure 4, except for the different placing of the parts,but the performance is quite different from the usual bridge circuitssince it does not operate on balance. The variation of Rm is readdirectly on the meter instead of being balanced out by variation of someother resistor. The current through the meter is almost equal to thecurrent of the measuring resistance Rm and much larger than in theresistance R5, a condition quite impractical for the conventionalWheatstone bridge in usual operation. A further difference from theusual bridge circuit is that the second variable resistance of thesystem, i. e., the ballast tube IU, varies primarily with line voltageand not meter-measured resistance, the variation being non-linear.Finally, and perhaps the most important distinction, is that the meteroperates as a straight current indicator of resistance exactly as inFigure 1. Thus, with R, equal to 500 ohms and with volts on the supplyline, when Rm equals ohms, the current through the meter is .349amperes, while with Rm equaling 200 ohms the meter current is .273amperes, which is exactly in inverse proportion to the variation of themeasured resistance Rm. While in these diagrammatic figures, Rm has beenrepresented as a resistance which it is desired to measure, it should,of course, be understood that the variation in resistance in thiselement may itself be responsive to variations in some othercharacteristic of material to be measured, whereupon'the meter readingbecomes a measure of the amount of this characteristic to be measured.It will be noted that this circuit is applicable to either direct oralternating current operation. It is of particular value in connectionwith alternating current Since with such current the meter does notdifferentiate between positive and negative current flow. In theordinary bridge balance is indicated by zero current through the meterand unbalance in either direction in the case of alternating currentflow would affect the meter reading in the same manner. In the presentcase the desired condition of full voltage compensation is accompaniedby a definite current flow through the meter and variations in the armto be measured in one or the other direction is accompanied by either anincrease or a decrease of fiow in accordance with the particulardirection of the variation. This is of particular importance whereresponsive means are to be employed since it is then essential thatdirection as well as amount of variation be taken into account.

Such a system is applicable not only to measure an electricalcharacteristic of a single element, but it is also applicable to themeasurement of the differences between the characteristics of twodifferent elements. This is illustrated, for example, in Figure 8, whichshows a circuit for measuring the differences of two inductances 30 and3i arranged in a bridge circuit with the resistors 32 and 33. It isfrequently desirable to operate such an inductance bridge at a frequencyhigher than is directly available from convenient sources. To supplythis higher frequency, there is shown an alternating current generator3l (supplying power to the bridge circuit) through the two leads 35 and36. The generator 34 has a separate direct current exciter I1 for itsileld coil 38 driven directly from the shaft I! of the generator 34.This exciter is provided with its own eld coil 40. At M is shown themeter which is connected through the lines 4I and 42, a resistance 43being interposed in line 42, to the iuli wave rectiiier systemcomprising the rectifiers 44 from which connections are made through thelines 45 and 4B with the opposite corners of. the bridge circuit betweenthe inductances 3l and 3| and between the resistances 32 and 33 so thata measuring direct current is imposed on the meter terminals. Thisarrangement of motor generator bridge circuit and meter does not, perse, constitute this invention.

With a generator and exciter oi' the type shown any variation in theshaft speed causes a somewhat greater proportionate variation in theexciter voltage and a considerably larger variation in the voltage ofthe generator 34. and withr a bridge circuit operating oiI balance as inthe diagram shown,- variation in the supply circuit voltage across thebridge circuit produces a proportionate change in meter reading. Thusany material variation in the shaft speed would cause a noticeablechange in the meter reading if it were not compensated for. It weassume, for example, that with normal shaft speed there is 100% voltageon the exciter and 100% voltage on the alternator, if the shaft speeddrops to' 90%, the exciter voltage drops to 85% and the gene- 'ratorvoltage to 75%, these variations being practically straight line. Inaccordance with the present invention, however, these eiects ofgenerator voltage on the meter reading are compensated for by directcurrent derived from the exciter through the leads 50 and 5l, in one ofwhich is positioned the resistor 52, these connections to the meterbeing such as to oppose variations in current derived from the measuringsystem due to voltage changes impressed on the bridge circuit. Assumingthat the meter is a one milli-ampere full scale meter and that ,normalreading is at midscale, or one-half a-milli ampere, and if theresistances 4I and 52 are so chosen that with normal voltage there is1.25 milli-.ampei'cs forward" current through the meter from the excitercompensator circuit through the lines 50 and 5| and a reverse current o!.75 nulli-ampere from the measuring circuit, then if the shai't speedfalls to 90%, the exciter voltage would fall to 85% so that thecompensating current fed from the exciter to the meter would fall to 85%or 1.0625 milli-amperes forward while the meter current from themeasuring circuit would fall to 75% or .5625 milli-ampere, giving ameter current oi .5 nulli-ampere; exactly the same as at 100% shaftspeed. Any variation in the inductances will be read as usual on themeter. With .25 nulli-ampere on the meter' from the measuring circuitthe meter will read full scale while with the 1.25 milli-amperes it willread zero, the scale being straight line. Exact compensation will exist,of course, only at midscale with the values chosen. The error at the topof` the scale will be one-half of what it would be with nocompensations, While at the bottom of the scale, which would have noerror without the compensation, the error with compensation will be thesame as at the top of the scale.

This system is reverse in some respects to the straight voltagecompensator circuit shown in Figure 4 in that the supply voltageproduces a change of voltage across the measuring system variableimpedance, and itis a voltage variation rather than an impedancevariation in the suplpy unit which is employed to produce thecompensation. 1

. Figure 9 iilustrates'an application of this method of voltagecompensation to a capacity measuring system operating at high frequency.In this system high frequency power is supplied to the measuring circuitfrom a 'vacuum tube oscillator. This oscillator is shown as suppliedwith alternating current power through the transformer $0,

one secondary coil 6i of whichfsupplies energy to the plate il! ot thethree-element vacuum tube il through a radio frequency vchoke impedanceI4, while a secondary of the transformer supplies current to the tubenlament 66. The plate circuit also contains the capacity 61 and theinductance 6I, which is couxzolcd to the inductance 89 of the gridcircuit, the grid being shown at 10. The grid circuit also contains thegrid condenser 1l and the resistor 12. At 13 is shown a tuning condenserfor the plate circuit. This circuit is coupled to a secondary circuitnearly in resonance therewith through a coupling Acondenser 14. Thissecondary circuit includes a measuring capacity 15 which may be maderesponsive to variations in some characteristics to ne measured, thissecondary including the coupled inductances 18 and 11, the capacity 1l,and a high frequency voltmeter comprising the milliammeter 19, therectiiler I0 and the limiting resistance 8|. This secondarycircuit-,also includes the tuning reactance l5. 'I'his circuit so fardescribed is not per se a part otf'this invention. Unless'some provisionwere made for compensation, the meter reading would vary not only withvariation of the characteristic measured, causing variation of thecapacity-1l, but also with any variation o! voltage supplied to thetransformer 60, and in accordance with this invention this variation inthe supply voltage is compensated for as will now be described.. s

With an oscillator o! the type shown in variation in the supply circuitvoltage causes a varia tion in the output of the oscillator of amagnitude more than proportional to the variation of the supply voltage,and `a variation of the' grid current of the oscillator of Veven greaterproportion that the output. Thus for a 10% increase in line voltage theoutput voltage to the oscillator may increase 15% and the grid currentmay increase 20%. Since the measuring circuit is coupled through aconstant impedance to the oscillator, the voltage across any point inthe measuring circuit, and specifically the voltage across the meter,varies in direct proportion to the voltage 'of the oscillator. Thus a10% increase of supply voltage would produce a 15% increase in theoscillator voltage which would result in a 15% increase inthe meterreading, other factors remaining constant. At the same time the gridcurrent would increase by 20%. As shown, therefore, in this ligure, aproportion of the grid current. derived from a shunt including theresistance tapped ofi from the grid resistor 12 at 83 iii and includingthe lead Il to the meter is passed through the meter in a directionopposite to that of the measuring circuit current. This shunt is sochosen, in the values of the resistors 8l and I2 and the position of thetap Il, that at nonmal" center scale operation of the meter the gridcurrent flowing through the meter is one and one half times the fullscale current, while the measuring circuit current to the meter is twicefull scale current, the difference between the two providing half scalecurrent and causing the meter deflection to be at midscale. If now thesupply voltage to the transformer vIl increases 10% there will be a 15%rise in the measuring circuit current which increases from 2.0 to 2.3

times full scale current. At the same time there will be a 20% increasein the current derived from the grid current shunt, going from 1.5 timesfull scale to 1.8 times full scale-current. The net current in the meterwill be thus 2.3 minus 1.8 times full scale current or 0.5 times fullscale current, as it was before the voltage impressed on the transformer60 changed. Thus at midscale the reading of the meter is unaffected.

. When with a normal or 100% applied voltage the current from themeasuring circuit decreases to 1.5 times full scale current due tochange in the characteristic being measured affecting the capacity ofthe condenser the current through the meter is 1.5 times full scalecurrent from the measuring circuit minus 1.5 times full s'cale'currentderived from the grid circuit, or zero cur# rent through the meter. Inthis case a 10% rise in applied voltage will cause the meter circuitcurrent to rise 15% as before or from 1.5 to 1.725 times full scalecurrent, the grid current applied to the meter rising from 1.5 times to1.8 times full scale current as before. Under these' conditions therewill be a change of .075 times full scale deflection for a 10% shift ofline voltage. Similarly when the characteristic of the material measuredchanges so that the meter current rises to 2.5 times full scale current,the net current through the meter will be exactly full scale curf rentproducing full scale deflection. Under such conditions a rise of 10% involtage will cause a. rise in measuring circuit current of .375 timesfull scale deflection and will produce the same rise of ,3 times fullscale deflection in the grid current, the difference producing a netriseor .075 times full scale deflection, which is the same as at zerometer scale but in the opposite direction.

'I'he circuit shown is employed for A null" measurement, that is, theapparatus is so set that when the characteristic measured is at itsdesired value the meter is at center scale, deviation from the desiredvaluey showing a deflection of the meter above or below the center scaleposition. It will be noted that the possible error due to line voltagefluctuation varies as the distance of the meter from center scale. Thus,the possible error due to line voltage variation varies exactly as theerror of the characteristic varies, a desirable arrangement since thegreater the deviation, the less the importance of the absolute accuracyof the measurement. It will be noted, however, that in the extreme casecf variation of 10% in the line voltage a variation of only '1l/2% inscale reading is produced.

Figure 10 shows a circuit similar to Figure 9, except that instead ofderiving the compensating meter current from the grid circuit, it isderived from a secondary winding 90 on the transformer 60 feedingthrough a choke @i and a rectifier B2 to ground through. the meter inopposition to the current derived from the meter circuit. By employingthe proper values of the various parts, it will be evident that exactcompensation of 'the meter reading at any desired point for variationsin line voltage supplied to the transformer 6B may be made similarly tothe way in which this .is done in the arrangement shown in Figure 9, ex-

cept that in Figure 10 the voltage variations derived from the secondary30 are equal to those of the supply and less than those of the measuringcircuit instead of being greater than those of the measuring circuit asin the arrangement of Figure 9.

- Figure l1 illustrates an application of the invention to a systemwherein the voltage compen sation is applied to a circuit which may beemployed for various purposes, this circuit being responsive tovariations in a desired controlling characteristic value independent ofvariations in the energy supply source. This circuit is in general thesame as that shown in Figure 8, except that a vacuum tube circuit issubstituted for the meter circuit of Figure 8. This vacuum tube circuitmay be used, `for example, .for initiating any desired of a greatvariety of controlling operations. In this Figure 11 the negative end ofthe resistor 52 is connected to the negative pole of the exciter and tothe bridge circuit between the impedances-32 and 33. The cathode |00 ofthe vac uum `tube 10i receives a negative direct current potential fromthe exciter through the slide |02 and the grid |03 of the tube iDi as analternating potential impressed thereon from between the impedanoes 30and 3i.

Under normal conditions, the negative potential from the exciter exceedsthe peak value of the alternating potential from the measuring circuitby an amount sufficient so that the most positive potential o'f the gridof the tube when the alternating potential from the measuring circuit isat peak is just sufficient to cause current flow in the plate circuit ofthe tube. Any decrease in the potential `from the measuring circuit,thus reduces the peak potential on the grid to a point where current inthe plate circuit does not flow, or in the case of a gas filledthermionic rectifier, the discharge is not started. Either with aconventional high vacuum tube or in the case of the gas filled rectifierthis flow of current and its interruption by a decrease of potentialfrom the measuring circuit can be applied to further controllingcircuits as may be desired.

In the metering circuits shown, for example, in Figures 8, 9, and 10, ithas been noted that the compensation is exact for only one meterreading, readings above and below this one reading being subject tovariations caused by variations in line voltage. Exact compensation forall instrument readings may, however, be produced by the use of certainadditional mechanism. One example of this is shown in Figure 12,the-circuit being substantiaily the same as that of Figure 9. Thetransformersecondaries Il and I! are not directly connected in Figure12.

In this figure, the oscillator plate current flows from the filamentcircuit at the point through a resistor iii back to the transformerwinding Il and to the tube plata $2 and. through the tube to thefilament B5. At the same time, the recti fier, in this case shown as atube H2, iiows a current in proportion to the voltage of theintermediate circuit. This current flows from the filament ill throughthe common point i2!) to the potentiometer H5 and back through theintemlediate coil l5 to the plate iii and grid i2! of the rectifiertube. A proportion of the voltage across the resistor IIB is picked upand opposes the potential through the resistor H6, the differenceactuating the galvanometer element lll. There is a mechanical connectionfrom the galvanometer element I I1 through a. mechanism I Il thegalvanometer lil which acts through the.

actuating mechanism iILmovingslider lll to the left on potentiometer IIIthus reducing the proportion oi the potential across the potentiometerwhich is picked up at lil. This action continues until the slider Illhas been moved to the lett a sumcient amount to cause the predeterminednow of current to pass through the galvanometer. lil. This mechanism Iper se being old and well known in the art. no detail descriptionthereof is deemed necessary.

- 'Ine reading of the circuit is taken by a read- -ao ing of theposition-ot the slider HI on the potentiometerv I Il as indicated by thescale shown.

- It will be noted that with any given line potential, the potentialacross theresistor Iii is a con -stant and the potential across the partof the 85 potentiometer ill picked up by slider lll is likevwise aconstant. If, as has been explained in the explanation of previousgures, the circuit con-y stants are predetermined in a fashion such thatthe current through the gaivanometer element o H1 is constant,regardless oi variation oi'line voltage for a particular scale reading,that is, assuming that the potential across the resistor H0 and thepotentialv across the potentiometer lil picked oil.' at the slider IIIvary by the same amount at a given current iiow through the`interm'ediate rectifier tube I'Ii, then this condition oi' exactcompensation will exist for all values oi' current through the rectiiierI Il, assumcil. as may be done, that the proportionate variation oicurrent through the tube IW with rents through this rectiner tube. Thatthis is the case is obvious, for an increase ot current ilow through theresistor i causes a reduction 5 non being auch um the net-potentialremand constant. y In all ci the illustratiodsioi the inventionhereinbefore given it will be noted that the potential and currentvariations in the system arising from variations of power supplied tothe system are opposed by other potential and current variations arisingfrom the same variations of power supply in such a manner that thevariations to be measured are alone registered by the meter. To that enda portion of the current ilow in the system, the proportion of whichisvariable with the supply voltage variation, is sliuntedl about the meterso that the remaining measuredcurrent is independent ot the supplyvoltage.

From the foregoing description oi' method and certain systems in whichit is shown as employed, it should be evident to those skilled in theart i that various changes and modiiications might i u be made withoutdeparting `from the spirit or line voltage remains constant for varyingcurof the' proportion pick up by slider Ill, this mo-l scope oi.' thisinventicnzas defined by the appended claims. v f

I claim:

1. In combination, first, an electrical including a controlling variableelectrical means and a mechanism responsive to variations oi' saidcontrolling means; second,-a source of power supplying energy to saidelectrical system third, power transmission means interposed betweensaid source and said system, said means having a variable characteristicresponsive proportionately to variations of said source; and fourth,means for feeding a predetermined. amount o! power from said sourcedirectly to said responsive mechanism, said amount of power beingpredetermined so that the variations thereof caused by variations ci.'said source counteract the variations which would be caused in saidresponsive mechanism by variations in said source acting through saidinterposed means.

2. In combination, nrst, an electrical system including a controllingvariable electrical means and a mechanism responsive to variations oisaid controlling means; second, a source or power supplying energy tosaid electrical system; third,

.power transmission means interposed between said source and saidsystem, said means having a variable characteristic responsiveproportionately to variations; of said source; and fourth, means forfeeding a current derived from said source directly to said responsivemechanism, said current varying due to variations in said source inamount equal and opposite to current variations reaching said responsivethrough said interposed means and caused by the' same variations in saidsource.

3'. In combination, smeasuring circuit including an impedance'to bemeasured and a meter responsive to the amount c! such impedance. asource of electric power, means interposed between said source andmeasuring circuit responsive in an electrical characteristic tovariations .in voltage oi'said source, and meam responsive to changes insaid characteristic for feeding power to said meter in directions tooppose changes in meter reading otherwise caused by .the same voltage'changes of said source acting l through said interposed meam.

4. In combination. a measuring circuit including avariable impedanceresponsive in its variations to variations in a characteristic 'to Vbemeasured and a meter responsive to the amoimt ot such impedance, asource o! electric power, v

means interposed between said source and measuring circuit responsive inan electrical characteristic to variations in voltage of said source,"and means also responsive in thesame charse- `teristic to voltagechanges in said source iortfeeding power to said meter in vdirections tooppose changesin 4meteiureadlng otherwise caused by the same voltagechanges oi' saidsourceacting through said interposed means.

` 6. In combination, a measuring circuit incliniing an impedance to bemeasured. a meter responsive tol the amount of such impedance,` a sourceor electric power connected to said measuring circuit, a voltageregulator between said source and measuring circuit, and a connection inparallel with said regulator from said source to said measuring circuit,and inlopposed relation to said meter oi the regulator-containingconnection, Iorimpressing on said meter energy opposed to that derivedfrom said regulatorcontaining connectionand variable with voltagechanges ot said source in amount to compensate ior energy changes due tothe same voltage changes impressed on said regulator-containingconnections.

6. In combination. a measuring circuit including an impedance to bemeasured. a meter responsive to the amount of such impedance, a sourceoi electric power connected to said measuring circuit, a ballast tubebetween said source and measuring circuit, and a connection in parallelwith said tube from said source to said measuring circuit, and inopposed relation to said meter of the tube-containing connection. forimpreming on said meter energy opposed to that derived from saidtube-containing connection and variable with voltage changes oi saidsource in amount to compensate for energy changes due to the samevoltage changes impressed on said tube-omtaining connections.

7. In combination with an alternating cur rent generator, a directcurrent exciter cmipledv for rotation with said geneatorand exciting thefield o! said generator, a bridge circuit containing a pair oiimpedancea the diierences between which are to be measured. said bridgecircuit being energixed from said generator, a direct current meter, andmeasuring connections including a rectiner between said bridge circuitand meter. oi a connection trom the output o! said exciter to said meterfeeding energy to said meter in amount and direction to compensate fordriving speed variations of said generator and exciter at apredetermined meter reading.

e. In combination with anl impedance to be measured. a measuring circuitincluding said impedance, a meter in said measuring circuit, anoscillator circuit for supplying energy to said measuring circuit. and asource of alternating em'rent power ior energizing said oscillatorcircuit. said oscillatory circuit including an electron tube having aplate and a grid, said measurins circuit deriving energy from the platecircuit, oi a connection from said grid circuit to said` meter forsupplying energy to said meter in amount and direction to oppose, byvariations therein due to variations in-voltage o! said source,variations caused by said source-voltage variations in .the energysupply to said meter derived through said measuring circuit.

- 9. In combination with an impedance to be measured.l a measuringcircuit including said impedanoe. a meter in said measuring circuit, anoscillatory circuit for supplying energy to said measuring circuit, anda source ot alternating current power for energizing said oscillatorycircuit, o! a connection from said source to said meter for supplyingenergy to said meter in amount and direction to oppose, by variationstherein due to variations in voltage oi' said source, variations causedby said source variations in the energy supply to said meter derivedthrough said measuring circuit.

10. In combinationv with an impedance to-ba measured. a measuringcircuit including lsaid impedance, a meter in said measuring circuit, anoscillatory circuit for supplying energy to said measuring circuit,andmeans including a transformer for supplying alternating currentoneri! to said measuring circuit, o! 's connection from said transformerto. said meter for supplyins energy thereto in amount and direction tosubstantially balance out in the reading oi! said meter variations dueto voltage variations in said transformer transmitted thereto throughsaid measuring circuit.

11. In combination with an impedance to he measured, a measuring circuitincluding said impedance. a direct current meter in said measuringcircuit, an oscillatory circuit for supplying energy to said measuringcircuit, and means including a transformer for supplying alwsnatingcurrent energy to said .measuring circuit, nia

.connection including a 'rectiiierhom said trans-y former to said meterfor supplying energy there# to in amount and direction to substantiallybalance out, in the reading of said meter. variations due to voltagevariations in said transformer transmitted thereto throughsaid measuringcircuit.

12. An electrical system suitable for alternating current andycomprising a bridge having an impedance in one arm variabiewith thecurrent passing therethrough, and having impedance in the other threearms of said bridge, said bridge including across connection betweentheiunctures of the-two arms o! each pair, said impedances being soproportioned that with a deiinite current flow in said cross connectionsaid current iiow is substantially unailected by voltage changesimpressed on the terminals of said bridge.

' i3. In a system for measuring a variable'impedance by measuringcurrent now, the method ot compensating for current variations arisingfrom variations in voltage applied tosaid. system, which comprisesshunting about a current measuring device in said system a portion ofthe current now in the system, the proportion shunted being so variablewith supply voltage that the remaining and measured current isindependent of supply voltage.

14. In a system of measuring a variable' impedance, the method oicompensating for. potential variation arising from variation oi powersupplied to the system. which comprises opposing said potentialvariation against further potential variation arising from saidvariation of power supply and having a predetermined relation theretosuch that the resultant diiierence while variable with variation in saidvariable impedance is independent o! said power supply variation.

WILFRED H. HOWE.

